A pressure sensor generally has a resin housing with which a conductive member as a terminal is insert-molded, and a sensing element mounted on the resin housing and electrically connected to the conductive member.
In the housing, the conductive member and the sensing element are covered with a protective member made of an electrical insulating material such as gel. For example, in a pressure sensor disclosed in U.S. Pat. No. 6,512,255 corresponding to JP-A 2001-304999, a protective member has a double-layer structure such that a second protective member is stacked on a first protective member. The first protective member made of fluorine-based rubber covers the conductive member and an electrical connection portion between the conductive member and a wiring connected to the sensing element. The second protective member made of fluorine-based gel covers the sensing element and a connection portion between the sensing element and the wiring connected to the conductive member. Generally, a bonding wire connects the conductive member and the sensing element. The first protective layer has a relatively high Young's modulus, i.e., high elasticity. The second protective member has a lower Young's modulus, i.e., lower elasticity than the first protective member.
It has been considered that the first protective member prevents air trapped between the conductive member and the housing from expanding and moving around in the protective member, because the first protective member having high elasticity covers the conductive member and the periphery thereof. Thus, reduction of insulating performance of the protective material may be prevented.
Further, the sensing element is covered with the second protective member having low elasticity so that pressure applied to the sensor is properly transmitted to the sensing element through the second protective member. Therefore, sensor characteristics of the sensor are adequately ensured.
Furthermore, the second protective member covers the connection portion between the sensing element and the bonding wire as a wiring, thereby preventing the bonding wire from being disconnected.
In the sensor described above, however, the second protective member made of fluorine-based gel is exposed at its top surface. Fluorine gel cannot tolerate under a high acid condition, PH of which is lower than 3, for example. Therefore, when the sensor is used in such a condition, the second protective member is corroded.
As a result of the corrosion, water may penetrate the second protective member so that elasticity of the second protective member may change. Accordingly, for example, variations may occur in pressure transmission to the sensing element through the second protective member, and the sensor characteristics may vary.